(1) Field of the Invention
This invention relates to a process for producing a reactive aqueous emulsion.
(2) Description of the prior art
Increasing social demands for the prevention of environmental pollution and the saving of resources have tended to restrict the use of organic solvents in recent years. Therefore, the availability of polymer emulsions using water as a medium has been noted in fields formerly using large amounts of organic solvents, such as coatings, adhesives and so forth. Various kinds of emulsions have been conventionally developed; however, these emulsions were produced by the heating emulsion polymerization method using water-soluble radical polymerization initiators (hereinunder referred to as "initiator-using polymerization"). Since the emulsion obtained by initiator-using polymerization contains low-molecular water-soluble compounds, such as a radical polymerization initiator, pH regulator, surface active agent and so forth, which adversely affect the properties of a film obtained from the emulsion and since the diameter of the particles of the emulsion is large, the emulsion lacks stability, and the denseness and smoothness of the film obtained are inferior. Therefore, the film obtained from the conventional emulsion possesses the disadvantages of low resistance to water, solvents and chemicals, and poor mechanical properties.
In order to eliminate the defects of the conventional polymer emulsion as stated above, an emulsion into which a reactive group is introduced and which can be crosslinked during the film-forming step has been proposed. For example, an emulsion containing a glycidyl group which reacts with a crosslinking agent to effect crosslinking in the film obtained from the emulsion to improve the properties of the film has been proposed in the initiator-using polymerization field. However, the disadvantage of low water-resistance caused by the large amount of low-molecular water-soluble compounds contained in the conventional emulsion could not be improved sufficiently even by the proposal and the proposal suffers in that the degree of crosslinking is lowered because of the inactivation of the glycidyl group in emulsion polymerization. When an unsaturated monomer containing a glycidyl group is used in amounts required for crosslinking between polymer particles of the emulsion, the degree of crosslinking of the interior of the polymer particles is extremely low and the film obtained suffers from defects in solvent-resistance, chemical resistance; etc.; on the contrary, when the unsaturated monomer is used in amounts large enough to crosslink the interior of the polymer particles of the emulsion, an excess of crosslinking between the polymer particles occurs and the film obtained suffers from defects in mechanical properties. In addition, the use of a large amount of unsaturated monomer is uneconomical.
On the other hand, since the ionizing radiation emulsion polymerization process requires no water-soluble radical initiator, pH-regulator and so forth, the emulsion obtained hardly contains any low-molecular water-soluble compounds and the water-resistance of the film obtained from the emulsion is excellent. However, the conventional ionizing radiation emulsion polymerization process does not improve the defects in the balance of the properties of the film caused by the distribution of the degree of crosslinking of the inter-polymer particles and the intrapolymer particles of the emulsion prepared by the initiator process as stated above.
A process for producing an aqueous emulsion which can form a film having excellent water-resistance, solvent-resistance, chemical-resistance and mechanical-resistance has, therefore, long been desired by those skilled in the art.